1. Field of the Invention
The invention generally relates to an optical film, and more particularly to a brightness enhancement sheet.
2. Description of Related Art
With the advancements in the optoelectronic industries in the recent years, the requirements for the exit beam shape provided by the light source module have gradually increased. Accordingly, various types of optical films have been developed by therequests of the beam shape. The optical films include diffusion sheets and brightness enhancement films (BEFs) such as prism sheets and lens arrays, and other sheets. More specifically, a prism sheet used as a BEF employs the light emitting prism surfaces of a plurality of rod-shaped prisms to select the incident light rays in different entrance directions. Generally, a light beam incident upon the bottom portion of the rod-shaped prisms at an angle of approximately 60° may pass through the prism due to refraction effects of the rod-shaped prisms, and the beam may exit the prism in a direction approximately perpendicular to the bottom portion of the rod-shaped prisms. Moreover, for light beams having incident angles that are significantly larger or smaller than 60°, the beams may not pass through the prisms due to the total reflection effect of the light emitting prism surfaces. These light beams are reflected back towards the optical device located below the prism sheet and reused.
Accordingly, the prism sheet achieves a light condensing effect for configuring the emitted light beams to be substantially forward-emitted.
However, a crest line at the top portion of the rod-shaped prism and a valley line formed between two adjacent rod-shaped prisms are clearly observable and they are disposed at regular intervals. Therefore, a moiré phenomenon is easily formed with a display panel (e.g., a liquid crystal display panel) disposed above the prism sheet or the BEF, thereby affecting the display quality of the display panel. Moreover, the rod-shaped prisms may form a sharp corner at the crest line, and the sharp corner may easily scrape or scratch the adjacent optical device. Additionally, since each of the rod-shaped prisms have the same height, the prism sheet may locally adhere to an adjacent optical film rather easily, thereby causing local abnormalities in the displayed image.
Taiwan Patent No. I302989 provides an optical film having a transparent substrate, a structural layer, and a plurality of light guide units. The structural layer may be a light condensing unit (e.g., a prism structure) disposed on the transparent substrate. The light condensing unit has a peak line and two surfaces. The light guide unit is disposed on the peak line or the two surfaces. The light guide unit may have a tapered shape, a circular arch shape, a circular arc shape or a prism shape.
Furthermore, Taiwan Patent No. M352033 provides an optical film having a microstructure layer, and the microstructure layer is a columnar structure. Taiwan Patent No. I274896 provides a light condensing sheet including a substrate and a plurality of curved combining prism units. Taiwan Patent Publication No. 200712655 provides an optical film with a surface having a plurality of irregular prism blocks. Taiwan Patent Publication No. 200819855 provides an optical film with a surface having a plurality of light guide microstructures, and each of the light guide microstructures has two or above peaks. U.S. Pat. No. 7,278,771 provides an optical film with a surface having curved rod-shaped prism structures. U.S. Pat. No. 6,845,212 provides an optical film with a surface having waved-shape rod-shaped prism structures.